1. Field of the Invention
The present invention relates to a surface-emitting laser element, a surface-emitting laser array, an optical scanning device, and an image forming apparatus.
2. Description of the Related Art
A vertical-cavity-type surface emitting laser (also referred to as “VCSEL” below) element is to emit light to a direction perpendicular to its substrate, has a lower price and lower electric power consumption, and is more compact, than an edge-emitting-type semiconductor laser element for emitting light to a direction parallel to its substrate, is suitable to a two-dimensional device, and has a high performance, and therefore has attracted attention recently.
For an application field of VCSEL, there are provided a light source (oscillation wavelength: 780 nm band) for a light-writing system in a printer, a light source (oscillation wavelength: 780 nm band and 850 nm band) for light-writing in an optical disk device, a light source for optical transmission system (oscillation wavelength: 1.3 μm band and 1.5 μm band) such as a LAN (Local Area Network) using an optical fiber, and the like. Furthermore, it has also been expected as a light source for optical transmission between boards, in a board, between chips of large scale integrated circuit (LSI), and in a chip of LSI.
In these application fields of VCSEL, it may frequently be required that light emitted from VCSEL (also referred to as “output light” below) has (1) a single wavelength, (2) a constant polarization mode, (3) a circular shaped cross-section of output light.
For example, a mirror or lens is used for focusing output light in a light-writing system and it may be necessary to conduct a precise and complex control of an optical path. In this case, because a mirror has a property of reflectivity that varies depending on the polarization direction of incident light and a lens has a property of a refractive angle that varies depending on a wavelength, it is preferable for output light to has a single wavelength and a constant polarization mode in order to suppress variation of a light intensity and blurring of a dot on a writing surface. Furthermore, it is desirable that the cross-sectional shape of a light beam irradiating a writing surface is circular in order to obtain a high definition quality of writing.
It is possible to evaluate the cross-sectional shape of a light beam by an FFP (Far Field Pattern), wherein the FFP strongly depends on the shape of an electric current passage area in an oxidized and narrowed structure. For example, in the case of a basic mode operation, the larger the width of an electric current passage area is, the narrower a FFP is, and the smaller the width of an electric current passage area is, the broader a FFP is. Therefore, it may be necessary for an electric current passage area to have a highly symmetric shape such as a square and a circle in order to a circular FFP.
Meanwhile, the oxidation rate in selectively oxidizing a layer to be oxidized selectively varies depending on its crystallographic orientation. For example, the oxidation rate of a layer to be oxidized selectively which is laminated on a substrate whose principal plane is a (1 0 0) plane (non-inclination substrate) has a 4-fold symmetry.
Therefore, it has been suggested that the outline of a mesa is adjusted in VCSEL manufacturing using a non-inclination substrate, in order to obtain an electric current passage area having a good symmetry such as a circle (For example, see Japanese Patent Application Publication No. 2007-142375 and Japanese Patent No. 3762765).
Furthermore, a special device may be required and various methods have been attempted in order to control a polarization mode, because the basic structure of VCSEL has a high symmetry (For example, see Japanese Patent Application Publication No. 09-172218, Japanese Patent No. 3799667, and Japanese Patent Application Publication No. 11-307882).
Moreover, it has been suggested that a so-called inclination substrate is used in order to a polarization mode (see IGA and KOYAMA, “Basic and Application of a Surface-Emitting Laser”, KYORITSU SHUPPAN CO., LTD, and A. Mizutam, N. Hatori, N. Nishiyama, F. Koyama and K. Iga, “A Low-Threshold Polarization-Controlled Vertical-Cavity Surface-Emitting Laser Grown on GaAs (311)B Substrate”, IEEE Photonics Technology Letters, Vol. 10, No. 5, 1998 MAY, p.p. 633-63).
However, the 4-fold symmetry of the oxidation rate in selectively oxidizing a layer to be oxidized selectively may be broken in VCSEL manufacturing using an inclination substrate. Therefore, the shape of an electric current passage area may be a polygon having a low symmetry such as a rectangle and an irregular ellipse, even for a mesa (mesa structure) whose cross-sectional shape parallel to a substrate has a high symmetry such as square, a regular polygon, and a circle. Thus, when the shape of an electric current passage area is a rectangle or an ellipse, the FFP of output light of a light source for writing may be elliptic and the shape of a light spot on a photoconductor surface may also be elliptic Therefore, there may be a disadvantage such that the fineness of writing may be degraded.